Against the background of climatic changes, it is a global aim to decrease the emission of pollutants into the atmosphere. This applies in particular to the emission of carbon dioxide (CO2), which accumulates in the atmosphere, impedes heat emission from the earth and thus, as a greenhouse effect, leads to an increase in the surface temperature of the earth.
Particularly in the case of fossil-fueled power plants for generating electrical energy or heat, a carbon dioxide containing flue gas is formed by the combustion of a fossil fuel. To avoid or decrease carbon dioxide emissions into the atmosphere, the carbon dioxide must be separated off from the flue gas. Correspondingly, in particular in the case of existing fossil-fueled power plants, suitable measures are being discussed in order to separate off the resultant carbon dioxide from the flue gas after the combustion (Post-Combustion-Capture). As a technical implementation, for this purpose, the carbon dioxide present in the flue gas is scrubbed out of the respective flue gas stream by an absorption-desorption process by means of a scrubbing medium, or of an absorption medium.
In addition to carbon dioxide, other acid gases, in particular nitrogen oxides (NOx) and/or sulfur oxides (SOx) are also absorbed in such, in particular, amine-containing and alkaline scrubbing media. In contrast to carbon dioxide, SOx, for example, forms with an alkaline scrubbing medium, inter alia, temperature-stable salts, such as, for example, sulfate. Thus, when a potassium-containing scrubbing medium is used, SOx precipitates out together with potassium as potassium sulfate (K2SO4), that can no longer be re-formed in a desorption unit. Owing to the change in alkali metal concentration caused thereby, these temperature-stable salts successively lower the capacity of the scrubbing medium for absorbing carbon dioxide. NOx is absorbed by the scrubbing medium, disproportionates in the solution and there forms nitrosamines, and also other soluble breakdown products.
In order to remove said contaminants, a two-stage processing of the scrubbing medium is customary. For this purpose what is termed a reclaimer is used with an SOx reclaiming stage for removing SOx or sulfates, and also with an NOx reclaiming stage for removing NOx, nitrite, nitrate and corresponding secondary products.
In the SOx reclaiming stage, that is to say the SOx reclaimer, potassium sulfate, for example, is separated out in crystalline form, on an industrial scale preferably by cooling crystallization. The soluble components are recirculated to the carbon dioxide separation process, and the potassium sulfate that is separated off is fed to a further use, for example as fertilizer additive or as feed stock for special chemical production. The NOx reclaiming stage, that is to say the NOx reclaimer, serves for recovering the active component of the scrubbing medium, that is to say, for example, an amino acid salt. For this purpose, the scrubbing medium is concentrated in an evaporation stage and from there is pumped into a crystallizer of the NOx reclaimer. Here, the amino acid salt is crystallized out as a solid. Soluble minor components are separated off as waste matter and fed to a corresponding disposal.
However, the use of the SOx reclaimer and the NOx reclaimer is not equally economical in every power plant type. Since the flue gases emitted by power plants differ greatly in their composition, in particular depending on the fuel used, according to the power plant type, different reclaiming stages are “dominant”. In coal power plants, even after a desulfurization of the flue gas, typically very high sulfur amounts are still present in the scrubbing medium, in such a manner that, for example, potassium sulfate can be separated out in crystalline form and subsequently sold. Correspondingly, the use of an SOx reclaimer, based, in particular, on a cooling crystallization, is economic in a coal power plant.
In gas power plants, the situation is somewhat different: the input sulfur amounts are markedly lower than in the case of coal power plants, in such a manner that the profits from the sale of crystallized potassium sulfate are negligible in comparison with the operating costs of an SOx reclaimer. In other words, the use of an SOx reclaimer is uneconomic in gas power plant applications.